Mammalian sialidases have been reported to give a great influence on a number of cellular functions including cell differentiation and cell growth by removal of sialic acids from glycoproteins and gangliosides. To understand the roles of the sialidases during development, we investigated expression pattern of three types of sialidase in developing rat brain and liver. For this purpose we cloned a new membrane-associated sialidase cDNA from rat brain. The cDNA encodes 418 amino acids containing three ASP-boxes characteristic of sialidases and the major transcript of 3.5 kb is highly expressed in brain and cardiac muscle but low in liver. Competitive polymerase chain reaction methods were developed to evaluate the mRNA level together with activity assays in comparison with cytosolic and lysosomal sialidases previously obtained. The results indicate that the expression of individual sialidase genes is spatiotemporally controlled with distinct roles in determining the concentration and components of sialo-glycoconjugates during development.